Fixed bug #18925: wrong results for queries with

subqueries on information schema that use MIN/MAX aggregation.

Execution of some correlated subqueries may set the value
of null_row to 1 for tables used in the subquery.
If the the subquery is on information schema it causes
rejection of any row for the following executions of
the subquery in the case when an optimization filtering
by some condition is applied.

The fix restores the value of the null_row flag for 
each execution of a subquery on information schema.

mysql-test/r/information_schema.result

@@ -1170,3 +1170,65 @@ f1()
 DROP FUNCTION f1;
 DROP PROCEDURE p1;
 DROP USER mysql_bug20230@localhost;
+SELECT t.table_name, c1.column_name
+FROM information_schema.tables t
+INNER JOIN
+information_schema.columns c1
+ON t.table_schema = c1.table_schema AND
+t.table_name = c1.table_name
+WHERE t.table_schema = 'information_schema' AND
+c1.ordinal_position =
+( SELECT COALESCE(MIN(c2.ordinal_position),1)
+FROM information_schema.columns c2
+WHERE c2.table_schema = t.table_schema AND
+c2.table_name = t.table_name AND
+c2.column_name LIKE '%SCHEMA%'
+        );
+table_name	column_name
+CHARACTER_SETS	CHARACTER_SET_NAME
+COLLATIONS	COLLATION_NAME
+COLLATION_CHARACTER_SET_APPLICABILITY	COLLATION_NAME
+COLUMNS	TABLE_SCHEMA
+COLUMN_PRIVILEGES	TABLE_SCHEMA
+KEY_COLUMN_USAGE	CONSTRAINT_SCHEMA
+ROUTINES	ROUTINE_SCHEMA
+SCHEMATA	SCHEMA_NAME
+SCHEMA_PRIVILEGES	TABLE_SCHEMA
+STATISTICS	TABLE_SCHEMA
+TABLES	TABLE_SCHEMA
+TABLE_CONSTRAINTS	CONSTRAINT_SCHEMA
+TABLE_PRIVILEGES	TABLE_SCHEMA
+TRIGGERS	TRIGGER_SCHEMA
+USER_PRIVILEGES	GRANTEE
+VIEWS	TABLE_SCHEMA
+SELECT t.table_name, c1.column_name
+FROM information_schema.tables t
+INNER JOIN
+information_schema.columns c1
+ON t.table_schema = c1.table_schema AND
+t.table_name = c1.table_name
+WHERE t.table_schema = 'information_schema' AND
+c1.ordinal_position =
+( SELECT COALESCE(MIN(c2.ordinal_position),1)
+FROM information_schema.columns c2
+WHERE c2.table_schema = 'information_schema' AND
+c2.table_name = t.table_name AND
+c2.column_name LIKE '%SCHEMA%'
+        );
+table_name	column_name
+CHARACTER_SETS	CHARACTER_SET_NAME
+COLLATIONS	COLLATION_NAME
+COLLATION_CHARACTER_SET_APPLICABILITY	COLLATION_NAME
+COLUMNS	TABLE_SCHEMA
+COLUMN_PRIVILEGES	TABLE_SCHEMA
+KEY_COLUMN_USAGE	CONSTRAINT_SCHEMA
+ROUTINES	ROUTINE_SCHEMA
+SCHEMATA	SCHEMA_NAME
+SCHEMA_PRIVILEGES	TABLE_SCHEMA
+STATISTICS	TABLE_SCHEMA
+TABLES	TABLE_SCHEMA
+TABLE_CONSTRAINTS	CONSTRAINT_SCHEMA
+TABLE_PRIVILEGES	TABLE_SCHEMA
+TRIGGERS	TRIGGER_SCHEMA
+USER_PRIVILEGES	GRANTEE
+VIEWS	TABLE_SCHEMA

mysql-test/t/information_schema.test

@@ -887,4 +887,37 @@ DROP FUNCTION f1;
 DROP PROCEDURE p1;
 DROP USER mysql_bug20230@localhost;
 
+#
+# Bug#18925: subqueries with MIN/MAX functions on INFORMARTION_SCHEMA 
+#
+
+SELECT t.table_name, c1.column_name
+  FROM information_schema.tables t
+       INNER JOIN
+       information_schema.columns c1
+       ON t.table_schema = c1.table_schema AND
+          t.table_name = c1.table_name
+  WHERE t.table_schema = 'information_schema' AND
+        c1.ordinal_position =
+        ( SELECT COALESCE(MIN(c2.ordinal_position),1)
+            FROM information_schema.columns c2
+            WHERE c2.table_schema = t.table_schema AND
+                  c2.table_name = t.table_name AND
+                  c2.column_name LIKE '%SCHEMA%'
+        );
+SELECT t.table_name, c1.column_name
+  FROM information_schema.tables t
+       INNER JOIN
+       information_schema.columns c1
+       ON t.table_schema = c1.table_schema AND
+          t.table_name = c1.table_name
+  WHERE t.table_schema = 'information_schema' AND
+        c1.ordinal_position =
+        ( SELECT COALESCE(MIN(c2.ordinal_position),1)
+            FROM information_schema.columns c2
+            WHERE c2.table_schema = 'information_schema' AND
+                  c2.table_name = t.table_name AND
+                  c2.column_name LIKE '%SCHEMA%'
+        );
+
 # End of 5.0 tests.

sql/sql_show.cc

@@ -3963,6 +3963,7 @@ bool get_schema_tables_result(JOIN *join)
         table_list->table->file->delete_all_rows();
         free_io_cache(table_list->table);
         filesort_free_buffers(table_list->table);
+        table_list->table->null_row= 0;
       }
       else
         table_list->table->file->records= 0;